This invention relates to processes of forming fiber composite structures. More particularly, this invention relates to processes for impregnating dry fiber preforms with resin. Still more particularly, this invention relates to processes for creating resin impregnated preforms that can be held in a pre-cured stage indefinitely.
Composite structures are made from dry fiber preforms into which are introduced uncured resins which are then cured to form the finished component or structure. These composite structures offer very high strength and low weight but at a comparatively high cost due to the expenses associated with material handling and processing technology and the exotic fiber and resin materials used. The need for expensive equipment such as autoclaves and vacuum/pressure pumps and the large amount of highly skilled labor involved in making these parts all add to the cost. Any cost savings that could be achieved in any of these areas would permit wider use of these high performance composite components and structures.
The basic ingredients for a composite part, dry fiber preform and resin, can be combined in a number of ways. The industry normally uses one of three processes, depending upon the particular application. These three processes are Resin Transfer Molding or Modification of RTM, Resin Film Intrusion, and Standard "B" Resin Impregnated on to fiber tow's or woven fabric. Resin Transfer Molding (RTM) is a process that impregnates a dry fiber preform located in a mold with resin that is heated in separate containers and is mixed and pumped into the mold under pressure. The resin and catalyst or hardener are usually stored in separate containers that are usually heated to obtain low viscosity. The resin is pumped from the two containers and is mixed and immediately injected, under pressure, into the mold to impregnate the fiber preform. The resin used is thereby catalyzed prior to introduction into the mold. As such, the resin has limited or finite flow characteristics when passing through multiple layers of fiber and often cannot successfully impregnate large, thick or complex-shape structures. This will result in voids within the cured composite part, thereby lowering its strength. This process uses considerable excess catalyzed resin that remains in the pressure pot, if pre-catalyzed, or in the injection lines, if in line mix is used. This excess resin requires careful and often costly clean up work. U.S. Pat. No. 5,403,537 to E. C. Seal et al. for "Method for Forming Composite Structures" and U.S. Pat. No. 5,500,164 to M. Livesay and W. Gallup for "Method and Apparatus for Producing Large Fiber Reinforced Structures" are representative of this method wherein resin is impregnated under pressure into a dry fiber preform mold, then the composite structure is final cured in the mold.
Resin Film Intrusion (RFI) is another method for resin impregnation into a preform. As with Resin Transfer Molding, the resin is already catalyzed when it is united with the dry fiber preform. RFI avoids the use of excess resin by casting the resin by itself to the "B" stage as slabs or films of resin. These films are cut to size, laid on the tool surface, and then the dry fiber near-net-shape preform is placed over the cast "film" of resin in the tool. This lay-up is then placed within a complicated vacuum bag assembly and cured under vacuum pressure in an oven or under positive pressure in an autoclave or heated press. This process often fails to insure complete and uniform resin impregnation, particularly when the "B" stage resin must flow into thick structures or long deep stiffened structures.
The third and most widely used process employs individual plies of tape, tow or fabric that have already been impregnated with "B" stage resin. Since the resin only needs to wet one or the adjacent ply, it can completely fill the voids in the fibers. However, the "B" stage cure for the resin necessitates holding the impregnated plies in 0.degree. storage prior to use. To form a part, the individual plies of tape, tow or fabric must be laid up by hand and then placed in a vacuum bag assembly. The lay-up is usually final cured in an autoclave. This process is very labor intensive, and the impregnated plies are troublesome to keep in cold storage and have a limited shelf life that carries with it attendant re-certification costs. U.S. Pat. No. 5,433,915 "Manufacturing Method of Composite Articles from Prepregs Which Avoids Internal Defects" is representative of an improved version of the complicated nature of this process wherein individual plies of pre-impregnated fabric are laid-up in molding jig of the matched die type, pressure is applied to the mold forcing out excess resin into a trap that is kept under back pressure to prevent micro cracks in the part.
For a different application, the Epoxylite Corporation, Irvine, Calif. has published Process Specification PS-43R which teaches a process for vacuum-pressure impregnation of resin into AC motors to provide a sealed insulation system capable of withstanding short term immersion. In this process, the stator of the motor is preheated to about 300.degree. F. and placed in a vessel where vacuum is pulled for 30-60 minutes while monitoring the temperature of the stator. The stator temperature must be at least 100-120.degree. F. immediately prior to the introduction of the resin into the vessel or there will not be successful impregnation. The vacuum in the vessel pulls in the resin until it covers the stator, and then pressure is introduced into the chamber to force the resin into the coils around the stator. The reference teaches that there is a critical relationship between the time of immersion, the temperature of the stator, and the viscosity of the resin. The relatively elevated temperature of the stator is necessary for the resin to penetrate it sufficiently. After the allofted time has passed, the pressure is released from the vessel, and the stator is lifted out of the resin, allowing the resin to wiped and otherwise drained off. The stator is then cured at 300.degree. F., and the process is repeated with another immersion cycle, followed by a final cure. The object of the process is to seal the windings on the stator, and the strength of the final product is not seemingly affected by the resin one way or the other.
Based on the above and foregoing, it can be appreciated that there exists a need in the art for a process for created resin impregnated preforms which overcomes the above-discussed drawbacks and shortcomings of the presently available technology. More particularly, there presently exists a need in the art for a process that utilizes a relatively low viscosity resin that will completely impregnate the dry fiber preform while avoiding the high labor hours and cold storage requirements associated with standard "B" stage resin impregnated fabric plies.
As will become apparent hereinafter, the present invention fulfills this need in the art by providing a cost effective method to introduce the precisely correct amount of low viscosity resin into a multi-layer dry preform with complete penetration of the resin into the preform, incur little or no waste of excess resin, and involve no cold storage requirements.